Patent GB-1015283-A (GB)
Title: Frequency synthesizer for single sideband communications
IPC: H03B21/04
Filed: 1962-09-19
Abstract: 1,015,283. Automatic frequency control; filters. AVCO CORPORATION. Sept. 19, 1962 [Oct. 2, 1961], No. 35671/62. Headings H3A and H3U.  In a variable frequency oscillator which has its output frequency stabilized by a reference frequency source, the output signal of the oscillator is mixed in successive stages with signals selectively derived from the reference frequency source to provide a signal at a final frequency which is applied to a frequency comparator with a further signal selectively derived from the reference source, the comparator provides an output signal to control the oscillator frequency. The reference source 24 operates at 7 Mc/s. and its output is applied to successive divider circuits 26, which divides by 7 to provide a 1 Mc/s. output, and 28, 30, 32 each of which divides by 10 to supply outputs at 100 kc/s. 10 kc/s. and 1 kc. respectively. The output of divider 26 is applied to a harmonic generator 34 which supplies output frequencies in 1 Mc. steps in the band 47-77 Mc/s:, the output of divider 28 is applied to harmonic generator 3 which supplies output frequencies in 100 kc/s. steps in the band 3À9-4À8 Mc/s., similarly harmonic generator 38 supplies frequencies in 10 kc/s. steps in the band 0À41-0À50 Mc/s and generator 40 supplies frequencies in 1 Kc/s steps 5À691-5À700 Mc/s. A programming device operated by the means for selecting the oscillator frequency applies D.C. signals to the filters 42, 44, 46, 48 which include voltage controlled capacitors whereby the filters select the required harmonics. The output of the oscillator 10 at a frequency which is approximately set by a voltage from the programming device applied via terminal 12 to a voltage controlled capacitor in the oscillator circuit, is applied via a buffer amplifier 50 to a mixer 52 which also receives an input from filter 42 and the mixer circuit output is applied in succession to mixer 56 in which it is mixed with the output of filter 44 and to mixer 60 in which it is mixed with the output of filter 46. The output of mixer 60 and the output of filter 48 are applied to a phase comparator circuit 22 which when the frequencies of its input signals differ supplies an A.C. output to unit 20 which in turn supplies a signal having an amplitude proportional to its input frequency to sweep the frequency of oscillator 10 towards the required value. As the oscillator frequency reaches the required value the output of unit 20 is cut off by damper unit 18 and the oscillator is controlled by the D.C. output of the discriminator. Filter units.-Each filter unit 42, 44 &c. comprises three sections A, B and C each including a coil 410, a fixed capacitor 411 and a voltage variable capacitor 412. The sections are coupled via a voltage variable capacitor 414 or 416 connected in series with a fixed capacitor 413 or 415 and the filter input terminal 419 is connected via a voltage variable capacitor 418 to the first section. Each voltage variable capacitor is connected directly or via one or more resistors 422-426 to a common point T to which an external D.C. control voltage is applied. Variation in the D.C. control voltage varies the frequency to which the filter is tuned, varies the coupling between sections to maintain a constant bandwidth and varies voltage variable capacitor 418 to maintain a constant input impedance to the filter.